Kinda a separate question, but is the on-use shield/absorb never considered? It's more stat points, and spread across two DR curves. I always see people defaulting to the defense relic when discussing the on-use ones. Curious why that is.

The main reason is because you get so much friggin' Shield rating as any kind of tank because the only stat it competes with is Accuracy Rating (which you can pretty much drop every time you see it because virtually no tank has any use for it). As such, the Shield rating portion of the use relic is hit monumentally hard by the DR curve (thanks to it being guaranteed to have the highest rating) and the Absorb portion suffers from the fact that, with 26/27 gear, Absorb rating has the worst DR (Defense has the smoothest curve and Absorb has the most extreme, going from very little DR to a lot of DR very quickly), not to mention that it's pretty standard to use the absorb proc relic, which pushes you pretty far into the realm of DR when both it and the use relic are active. In short, it's because Shield/Absorb are hit harder than Defense by the DR curve.

Of course, a lot of tanks have begun to abandon the use relics because the developers saw fit (for some unknown reason) to reduce the durations of the use effects from 30 seconds to 20 seconds (but only for the tank relics, the DPS/healer relics are all still 30 seconds...). It's a sad but true fact that you actually get better average rating contributions out of the Rakata use relics for tanks than you do out of the DG or campaign use relics (with the exception of the Campaign Shield/Absorb relic which has a 30 sec duration whereas the Defense relic has a 20 sec relic, which has perplexed me even more than the fact that the developers reduced the duration of both of the DG tank relics).